1. Field of the Invention
The present invention generally relates to a stereophonic sound image enhancement apparatus and a stereophonic sound image enhancement method, capable of enhancing a stereophonic sound image during a stereophonic sound reproducing operation. The apparatus and methods may be used in, for example, electronic music instruments, game machines, and acoustic appliances (for example, mixers). More specifically, the present invention is directed to a technique for enhancing stereophonic sound images during a 2-channel speaker reproducing operation.
2. Description of the Related Art
Several conventional sound image localizing techniques are known in this field. For example, in one technique, a left channel signal and a right channel signal for a stereophonic sound are produced and supplied to left/right speakers, respectively, to produce stereophonic sounds simultaneously so that a sound image is localized. Essentially, this conventional sound image localizing technique localizes the sound image by changing the balance in the sound volumes of the left/right channels. As a consequence, the sound image is localized only between the left speaker and the right speaker.
Another sound image localizing technique has been developed where a sound that a phase of a right-channel signal is inverted and is mixed with a left-channel signal and a phase of the left-channel signal is inverted and is mixed with the right-channel signal. As a consequence, the resulting sound image is localized at any position except for positions between the left speaker and the right speaker (namely, a left side, or a right side located apart from left/right speakers). This sound image localizing technique is disclosed in, for instance, xe2x80x9cSOUND IMAGE MANIPULATION APPARATUS AND METHOD FOR SOUND IMAGE ENHANCEMENTxe2x80x9d of WO94/16538 (PCT/US93/12688).
This conventional sound image manipulation apparatus/method for sound image enhancement produces a difference signal between a left-channel input signal and a right-channel input signal. The amplitude or magnitude of this difference signal is adjusted, and the adjusted difference signal is supplied to a band-pass filter. Then, the difference signal filtered by the band-pass filter is added to the left-channel input signal to produce the left-channel output signal. Similarly, the difference signal filtered from the band-pass filter is subtracted from the right-channel input signal to produce the right-channel output signal. The left-channel output signal and the right-channel output signal are supplied to the left speaker and the right speaker, respectively. According to the conventional sound image manipulation apparatus and sound image enhancement method, the sound image can be localized at any position except for positions between the left speaker and the right speaker. As a consequence, the stereophonic sound image is enhanced and a sound stage having excellent presence may be realized.
However, these sound image manipulation apparatus and sound image enhancement methods may have a problem in that when the enhancement effect of the stereophonic sound image is increased by controlling the amplitude of the difference signal the sound quality may be deteriorated. In the worst case, the sound quality would be deteriorated to such an extent that the inputted source could not be reproduced.
Also, the Schroeder method is known in this field as another technique capable of localizing the sound image at any position except for the position between the left speaker and the right speaker. In the Schroeder method, crosstalk sounds from the left speaker to a right ear and from the right speaker to a left ear are canceled. As a result, a listening condition using a headphone may be established. When the Schroeder localizing technique is introduced, the sound image can be localized at any arbitrary position such as positions immediately beside a listener, immediately behind a listener, and also between the left speaker and the right speaker.
However, if a sound image localization apparatus to which the basic idea of this Schroeder method has been strictly applied is constituted by an analog circuit, then a huge amount of hardware is necessarily required. On the other hand, if this sound image localization apparatus is arranged by a digitally-operated processor such as a digital signal processor (DSP) and a CPU, then a large amount of data processing operation is required. As a result, conventionally, the sound image localization apparatus with employment of the Schroeder method is allowed to be applied only to such a limited appliance, for instance, high-grade electronic musical instruments, game machines, and acoustic appliances.
As a consequence, the present invention has an object to provide a stereophonic sound image enhancement apparatus and a stereophonic sound image enhancement method, capable of enhancing a stereophonic sound image without deteriorating a sound quality during a 2-channel speaker reproducing operation. Furthermore, another object of the present invention is to provide a stereophonic sound image enhancement apparatus and a stereophonic sound image enhancement method, which can be made by a simple circuit arrangement and at low cost.
To achieve the above explained object, as indicated in FIG. 1, a stereophonic sound image enhancement apparatus, according to a first aspect of the present invention, includes:
a first all-pass filter 10a for changing a phase of a left channel input signal Lin in response to a frequency of the left channel input signal Lin to thereby output a phase-changed left channel input signal;
a second all-pass filter 10b for changing a phase of a right channel input signal Rin in response to a frequency of the right channel input signal Rin to thereby output a phase-changed right channel input signal;
first calculating means 11a for calculating a first difference between the left channel input signal Lin and the phase-changed right channel input signal outputted from the second all-pass filter lob to thereby output a first difference signal corresponding to the first difference as a left channel output signal Lout; and
second calculating means 11b for calculating a second difference between the right channel input signal Rin and the phase-changed left channel input signal outputted from the first all-pass filter 10a to thereby output a second difference signal corresponding to the second difference as a right channel output signal.
Each of the first all-pass filter 10a and the second all-pass filter may comprise by a first order all-pass filter. In general, this first order all-pass filter may not change the frequency characteristic of the input signal, but will change the phase characteristic thereof. For example, as indicated in FIG. 2, such a filter may be employed, by which the phase of the input signal is shifted by 180 degrees.
Each of the first calculating means 11a and the second calculating means 11b comprises, for example, an operational amplifier.
The first calculating means 11a subtracts the left channel input signal Lin from the phase-changed right channel input signal derived from the second all-pass filter 10b to obtain a first difference signal which is outputted as the left channel output signal Lout.
Similarly, the second calculating means 11b subtracts the right channel input signal Rin from the phase-changed left channel input signal derived from the first all-pass filter 10a to obtain a second difference signal which is outputted as the right channel output signal Rout.
Now, a consideration is made of such a case that both the first all-pass filter 10a and the second all-pass filter 10b are not employed. In this case, the first calculating means 11a subtracts the left channel input signal Lin from the right channel input signal Rin to obtain a difference signal, and then outputs this difference signal as the left channel output signal Lout. Similarly, the second calculating means 11b subtracts the right channel input signal Rin from the left channel input signal Lin to obtain another difference signal, and then outputs this difference signal as the right channel output signal Rout.
When sounds are produced based on the left-channel output signal Lout and the right-channel output signal Rout, lower sound ranges of the sounds are attenuated. The reason for this attenuation is as follows. Generally speaking, an audio signal (constructed of left channel input signal Lin and right channel input signal Rin) reproduced from a musical medium is processed in such a way that a listener can hear low-range-sounds of musical instruments such as a bass and a drum from a center position between a left speaker and a right speaker. This implies that the low sound range components contained in the audio signal in the left channel and the right channel have frequency characteristics similar to each other. As a consequence, when the left channel input signal Lin is subtracted from the right channel input signal Rin, the low sound range components substantially disappear. That is, the low sound ranges are attenuated.
To the contrary, as explained in the stereophonic sound image enhancement apparatus according to the first aspect of the present invention, the subtracting calculation comprises the difference between the input signal of one channel and the input signal of the other channel which has been filtered by the all-pass filter, so that the left channel input signal Lin and the right channel input signal Rin are produced. As a result, the attenuation in the low sound range can be avoided. This is because, as indicated in FIG. 2, the first order all-pass filter shifts the phase of the input signal by 90 degrees around the cut-off frequency xe2x80x9cfcxe2x80x9d, and further shifts this phase by approximately 180 degrees (namely, reverse phase) while the frequency thereof is lowered. Conversely, this first order all-pass filter shifts the phase of this input signal by 0 degree (namely, normal phase) while the frequency thereof is increased. In other words, as to the first order all-pass filter, there is such a trend that the phase of the input signal is negatively inverted at frequencies lower than the cut-off frequency fc, so that the shifted phase of this input signal is outputted as the negative value. Conversely, there is another trend that the phase of the input signal is positively inverted at frequencies higher than the cut-off frequency, so that the shifted phase of this input signal is outputted as the positive value.
Accordingly, in the first calculating means 11a and the second calculating means 11b, the adding calculation is essentially carried out for the right/left channel input signals at a frequency range lower than the cut-off frequency fc, whereas the subtracting calculation is essentially carried out for the right/left channel input signals at a frequency range higher than the cut-off frequency fc. As a consequence, there is no possibility that the respective low sound range components contained in the left channel input signal Lin and the right channel input signal Rin are canceled by each other in the subtracting calculation. As a consequence, musical sounds with better sound qualities can be produced without attenuating the low sound ranges.
It should be noted that the transfer function of the first order all-pass filter is expressed by the following formula (1):                               G                      (            s            )                          =                              s            -                          ω              a                                            s            +                          ω              a                                                          formula        ⁢                  xe2x80x83                ⁢                  (          1          )                    
where symbol xe2x80x9cxcfx89axe2x80x9d=2xcfx80f, symbol xe2x80x9csxe2x80x9d is Laplace operator, and phase angle xe2x80x9cxcex8xe2x80x9d=xe2x88x922 tan31 1(xcfx89/xcfx89a).
Also, as indicated in FIG. 3, a stereophonic sound image enhancement apparatus, according to a second aspect of the present invention, further includes:
first delay means 12a for delaying the first difference signal derived from the first calculating means 11a to thereby output a delayed first difference signal as a third difference signal;
third calculating means 14a for subtracting the left channel input signal Lin from the third difference signal derived from the first delay means 12a to obtain a difference signal which is outputted as a left channel output signal;
second delay means 12b for delaying the second difference signal derived from the second calculating means 11b to thereby output a delayed second difference signal as a fourth difference signal;
and fourth calculating means 14b for subtracting the right channel input signal Rin from the fourth difference signal derived from the second delay means 12b to obtain another difference signal which is outputted as a right channel output signal.
Both the first delay means 12a and the second delay means 12b produce an inter aural time difference. In the case that these first delay means 12a and second delay means 12b comprise a digital circuit, these delay means may be arranged by employing a delay buffer for delaying the input signal by a software process operation. The delay buffer, may comprise a cycle buffer which can write the data, while cycling within a preselected storage region.
On the other hand, when the first delay means 12a and the second delay means 12b comprise an analog circuit, these first/second delay means may comprise a first order all-pass filter or a second order all-pass filter, which functions as a group delay equalizer. This group delay equalizer ideally owns a flat group delay characteristic, which does not depend upon a frequency (see broken line shown in FIG. 4). However, as the frequency is increased, the large group delay is difficult to achieve in the analog circuit. On the other hand, it has been recognized that if the group delay is equalized up to approximately 2 kHz, then a sufficient sound image enhancement effect could be achieved. As a result, as this group delay equalizer, a group delay equalizer capable of realizing a group delay of, for example, approximately 180 xcexcs corresponding to the inter aural time difference may be employed.
As one example, a formula (2) indicative of the group delay equalizer of 180 xcexcs is expressed as follows:                                           G            2                    ⁡                      (            s            )                          =                                            s              2                        -                          2              ⁢              ζ              ⁢                              xe2x80x83                            ⁢                              ω                0                            ⁢              s                        +                          ω              0              2                                                          s              2                        +                          2              ⁢              ζ              ⁢                              xe2x80x83                            ⁢                              ω                0                            ⁢              s                        +                          ω              0              2                                                          formula        ⁢                  xe2x80x83                ⁢                  (          2          )                    
where symbol xe2x80x9cxcfx890 xe2x80x9d is an angular frequency at which the phase becomes 180 degrees, symbol xe2x80x9cxcex6xe2x80x9d denotes an attenuation ratio (xe2x80x9cxcex6xe2x80x9d=xc2xdQ), and symbol xe2x80x9cxcex6xe2x80x9d represents Laplace operator (jxcfx89).
A solid line of FIG. 4 shows such a group delay characteristic of the first delay means 12a and the second delay means 12b when the angular frequency xcfx890 is selected to be approximately 3 kHz, and the attenuation ratio xe2x80x9cxcex6xe2x80x9d is equal to 1 in the above-described equation (2). As is apparent from the graphic representation of FIG. 4, the substantially ideal group delay characteristic may be achieved up to about 2 kHz.
The inter aural time difference produced by the first delay means 12a and the second delay means 12b may constitute a major function so as to obtain the sound delay characteristics. Assuming now that these first delay means 12a and second delay means 12b are not employed, it may be possible to obtain sound delay characteristics to a certain extent. However, since the stereophonic sound image enhancement apparatus is equipped with these first delay means 12a and second delay means 12b, very large delay characteristics may be obtained. It should be noted that the sound image localizing/enhancing technique using the inter aural time difference produced by the first delay means 12a and the second delay means 12b is disclosed in U.S. Pat. No. 6,035,045, filed Oct. 17, 1997, by Akihiro Fujita, Kenji Kamada, and Kouji Kuwano, entitled xe2x80x9cSOUND IMAGE LOCALIZATION METHOD AND APPARATUS, DELAY AMOUNT CONTROL APPARATUS, AND SOUND IMAGE CONTROL APPARATUS WITH USING DELAY AMOUNT CONTROL APPARATUSxe2x80x9d in which priority is claimed based on Japanese Patent Application No. Heisei 8-298081. The disclosure of the above U.S. Pat. No. 6,035,045 is incorporated herein by reference.
The above-explained third calculating means 14a and fourth calculating means 14b may be constructed of, for instance, operational amplifiers. The third calculating means 14a is arranged to subtract the left channel input signal Lin from the delayed signal from the first delay means 12a and output the subtracted signal as a left channel output signal Lout. Similarly, the fourth calculating means 14b is arranged to subtract the right channel input signal Rin from the delayed signal from the first delay means 12b and output the subtracted signal as a right channel output signal Rout. The crosstalk components can be removed from the left channel input signal Lin and the right channel input signal Rin by the third calculating means 14a and the fourth calculating means 14b. 
When sounds are produced using the left channel output signal Lout and the right channel output signal Rout produced in the above-explained manner, since the sound image can be localized at any position except for such a position between the left speaker and the right speaker, it is possible to obtain sound images extended to a further wide spreading range around the listener, as compared with the above-described stereophonic sound image enhancement apparatus according to the first aspect of the invention.
Also, as indicated in FIG. 5, a stereophonic sound image enhancement apparatus, according to a third aspect of the present invention, further includes:
first attenuating means 13a for attenuating the third difference signal derived from the first delay means 12a to supply an attenuated third difference signal as a fifth difference signal to the third calculating means 14a; and
second attenuating means 13b for attenuating the fourth difference signal derived from the second delay means 12b to supply an attenuated fourth difference signal as a sixth difference signal to the fourth calculating means 14b. 
Both the first attenuating means 13a and the second attenuating means 13b may comprise, for example, a variable resistor. In accordance with this arrangement, since the attenuation ratios in the first attenuating means 13a and the second attenuating means 13b may be varied, the spreading degree of the stereophonic sound image can be changed.
Also, a stereophonic sound image enhancement method, according to a fourth aspect of the present invention, comprises the steps of:
changing a phase of a left channel input signal in response to a frequency of the left channel input signal to output a phase-changed left channel input signal;
changing a phase of a right channel input signal in response to a frequency of the right channel input signal to output a phase-changed right channel input signal;
calculating a first difference between the left channel input signal and the phase-changed right channel input signal to output a first difference signal corresponding to the first difference as a left channel output signal; and
calculating a second difference between the right channel input signal and the phase-changed left channel input signal to output a second difference signal corresponding to the second difference as a right channel output signal.